Add environment variables YAML config for Applications

fixes #569 
add optional "environment" key to application YAML config
a dictionary of environment variable mappings to be made
available to the application's container
And I think we may as well do a try: except here and raise a nice ConfigurationError if the value isn't unicode....but I don't think we should just cast to unicode...that'll just trigger __repr_. Better to test the type and raise a ConfigurationError.